i2c: sh_mobile: devm conversion, irq setup

This is what devm was made for. No rollback mechanism needed, remove the
hook parameter from the irq setup function and simplify it. While we are
here change some variables to proper types.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>